Microsoft have released IE9 Platform Preview 3, an application that gives developers access to the IE9 rendering engine (it's not a full browser). In this update they have added hardware accelerated HTML5 Video, Canvas, Fonts (using WOFF) and big improvements in JavaScript with ES5, DOM Traversal, L2 and L3 events and 83/100 Acid3 score. It sits between Firefox and Chrome 6 on JavaScript speed, but outperforms every browser in real tests.

Any idea if they are going to support HW acceleration under Linux (or other OpenGL-supported platforms) and if so, when? Or will this again be a Windows-only feature?

it is not crystal-clear from this post, but it seems to indicate that hardware acceleration for Firefox is ordinarily via openGL, and optionally via Direct2D can be enabled instead for those people on Windows whose system's support for openGL is abysmal.